Connections clean?
Anything plugged into the cigaret outlets? Phone charger etc. Doors completely closed, no dome lights, hood/truck lights on or anything of the sort for a potential draw?

How old is the battery?
Does the battery have a full and complete charge? A small and short charge is not sufficient enough and the battery can be weak and losing full charging capabilities. May also have internal issues as well.

If not, this could be a parasitic draw issue and you'll need to track it down.

Check the above, if you need help finding a possible drain in your vehicle, and you have a VDOM here is two helpful links. However, Proceed with caution, shorting out your leads could pose harm to your OBC.

Use Caution:
BMWs are famous for their long shut-down times and purely from my own experience I expect somewhere between 16 and 18 minutes before sleep mode is reached. This wait-time is a real headache when trying to fault-find because, until you have allowed enough time to expire, you could end up chasing a problem that doesn't exist. Furthermore, start tracing too early by pulling or replacing circuit fuses and you're likely to reset a controller, adding more wait-time.